1. Field of the Invention
The present invention relates generally to electron guns and more particularly to plasma-cathode electron guns.
2. Description of the Related Art
A plasma-cathode electron gun was disclosed in U.S. Pat. No. 4,912,367 issued Mar. 27, 1990 in the name of Robert W. Schumacher et al., and assigned to Hughes Aircraft Company, the assignee of the present invention. The electron gun operated with a pulsed discharge in an ionizable gas contained in a hollow cathode enclosure. This discharge produces a uniform plasma of electrons and positive ions. The electron beam pulse was extracted from the plasma by a beam voltage impressed between a discharge grid and an anode positioned adjacent an outlet of the enclosure.
The above cited patent also described an exemplary application of the plasma-cathode electron gun in which its electron beam was injected into a slow-wave structure that operates in the presence of a low-pressure ionizable gas. The slow wave structure reduces electromagnetic phase velocity so as to match the speed of the electron beam. Space-charge waves on the beam can then be resonantly coupled to waveguide modes in a process that transfers energy from the electron beam to a microwave signal that is subsequently coupled out of the slow-wave structure.
The electron beam is confined and transported through the slow-wave structure by electron beam ionization of the gas surrounding the slow-wave structure to produce ions that neutralize the beam and prevent space charge blowup. A magnetic confining force is produced by the axial beam current which produces an azimuthal magnetic field directed back upon the beam to generate thereon a radially inward-directed force. Backflowing ions from the slow-wave plasma are harmlessly absorbed by the plasma cathode.
In this exemplary application, gas pressure in the slow-wave structure must be above a minimum required to produce sufficient plasma to control space-charge blowup and below a maximum that causes the plasma to short the slow-wave structure. This pressure range, typically positioned below 5.times.10.sup.-4 Torr, has generally been found to be below the pressure range required for optimum operation of the cold-cathode discharge in the plasma-cathode electron gun, e.g., 5.times.10.sup.-3 Torr.
This pressure differential conflict has been addressed by coupling a transient injection system to the electron gun. This system injects sufficient gas into the enclosure to strike the cold-cathode discharge. The injection is timed so that the beam pulse is past before the injected gas diffuses into the slow-wave structure region.
This transient system can be formed around a gas-puff valve coupled to the plasma-cathode enclosure. The valve is connected to a gas-puff supply and the timing coordination between the discharge pulse and the gas-puff valve is achieved via a fiber optic light link. A portion of the injected gas diffuses through the enclosure outlet into the slow-wave structure which must be pumped back down to the preferred slow-wave pressure range before the next pulse is initiated. Thus, not only does the transient system involve the addition of considerable hardware with consequent size and weight increase but the system pulse repetition rate is limited, e.g., typically to less than 100 Hz, by the injection and pumping functions. A more detailed description of the transient gas injection system may be found in Goebel, Dan M., et al., Proceedings 9th International Conference on High-Power Particle Beams, Washington, D.C., May 25, 1992, pp. 1093-1098.
This paper also describes a mesh coupled to the discharge anode to define a plasma face. Definition of this face helps to insure that the electrons enter an adjacent acceleration region from the same location independent of the system voltage and current. This reduces variations in the system's beam optics. The mesh also stabilizes the electron beam extraction by providing a measure of isolation between the plasma discharge process and the electron beam extraction process.
Another electron gun structure is described by S. Tanaka, et al. (see S. Tanaka, et al., "Design and experimental results of a new electron gun using a magnetic multipole plasma generator", Review Scientific Instruments, American Institute of Physics, March 1991, pp. 761-771).
This structure includes a plasma generator chamber coupled to a three grid accelerator. The plasma generator has a copper chamber with Sm-Co permanent magnets attached to its outer surface to form a magnetic multipole configuration in the chamber. The chamber is preferably filled with hydrogen with tungsten filaments inserted therein.
The accelerator is composed of three grids, respectively called plasma grid, gradient grid and earth grid, which are held in alumina ceramic insulators. The gradient grid is disposed between the plasma and earth grids with the plasma grid spaced closest to the chamber. Various aperture configurations are disclosed for each of these grids.
The plasma and gradient grids are negatively biased with respect to the earth potential. The potential of the gradient grid is set between the plasma grid potential and earth potential. A ration of gradient grid potential to plasma grid potential is defined. By varying the ratio, it is stated that the electron beam optics can be controlled for a given combination of acceleration voltage and beam current.
As reported in the above paper, the gun structure described therein is limited to a beam current a 4 amperes which is obtained in a gas pressure environment of approximately 1.times.10.sup.-3 Torr. In addition, over 8 kW of discharge power was required to produce 2 ampers of beam current for an efficiency of less than 0.25 A/kW. This beam current is not compatible with high power microwave tube requirements and the gas pressure is not compatible with a low gas pressure required in the slow-wave structures described above.
Other reference s directed to plasma-cathode structures include U.S. Pat. No. 3,831,052 issued Aug. 20, 1974 in the same of Ronald C. Knechtli and assigned to Hughes Aircraft Company, the assignee of the present invention. This patent disclosed an electron gun directed to ionization of the gas in a laser cavity. The plasma of the electron gun is produced by a high pressure (&lt;1.times.10.sup.-2 Torr) glow discharge. Accordingly, electron guns in accordance with this patent are not compatible with the low gas pressure required in the slow-wave structures described above.